In this application, the applicants carefully addressed the important application of the proposed SPECT technique in breast cancer staging and characterization. Preliminary results from a prototype RMSSH collimator were included. Also, a new specific aim and methods to conduct an initial clinical trial of the proposed technique were added. The innovations of the proposed SPECTM technique include: (1) the use of a MSSH collimator that provides 3 to 4 times the detection efficiency within its field-of-view than a conventional collimator with the same spatial resolution, (2) the ability to acquire complete sets of projection data by simply rotating the MSSH at 2 to 3 positions on a 90 degree arc around the breast, (3) the ability for the RMSSH collimator to stay close to the breast during data acquisition for best spatial resolution, and (4) the use of fully 3D reconstruction methods that reconstruct the complete projection data sets to provide high quality, quantitatively accurate and artifact-free reconstructed images. The research design and methods include: (1) the design of special optimized MSSH collimators for the SPECTM technique, (2) the development of transmission and emission data acquisition strategies for complete projection data sets from using the rotating MSSH collimators, (3) the development of fully 3D reconstruction methods that provide quantitatively accurate reconstructed images of the breast, (4) evaluation of the 3D SPECTM images for the detection and characterization of small and low contract breast lesions, and (5) initial pilot clinical evaluation of the SPECTM technique in five patients. The proposed new SPECTM technique can be easily implemented in most nuclear medicine clinics requiring only a low cost (about $20,000) RMSSH collimator assembly. It will provide much improved capability in evaluating the extent and characterization of breast cancers for staging purposes that have been found useful in the surgical management of breast cancer.